The present invention is related to the field of pulp production, more particularly the invention relates to the field of refining wood chips into pulp for paper manufacturing.
Single and double disc refiners are well-known in the art of pulp production. Such refiners are typically employed in the production of pulp from lignocellulose-containing fiber material, in a two-step process having primary and secondary refining. In a thermomechanical pulping (TMP) process, wood chips are fed into a pressurized pre-heater by a first plug screw feeder or first rotary valve and preheated with steam. A second screw conveyor or second plug screw feeder then discharges the chips from the pre-heater. A ribbon or other feeder then moves the preheated chips into a refiner for initial refining into pulp. Should a plug screw feeder be used for the second feeder, the system pressures in the pre-heater and refiner can be decoupled. The pulp from the primary refiner is then introduced into a secondary refiner for further processing.
Refiners have conventionally been operated at pressures of approximately 30-55 psi (207-345 kPa) and speeds of 1500 to 1800 rpm for single disc refiners end 1200 to 1500 rpm for double disc refiners. To produce pulp of desired quality, the wood chips are mixed with steam and retained in the pre-heater at a predetermined temperature and pressure prior to primary refining. The time of retention, or residence time, directly effects pulp quality. Residence time is the time the chips are maintained between the first plug screw feeder and the refiner feeder. In a decoupled system, a residence interval exists in the pre-heater and also from the second discharge plug screw feeder to the refiner feeder. Each of these two residence intervals can be regulated at a different pressure. The conveying and refining time for the chips to be moved by the refiner feeder into the refiner and through the refiner discs is not factored into the residence time. The reason is the short duration of the conveying and refining time. For most refiners, the conveying and refining time is less than one second.
An important factor in the competitiveness of disc refiners with other methods of pulp refining is the energy consumption necessary to operate the disc apparatus. Rapid increases in energy cost can render disc refiners non-competitive against other forms of pulp production from an economic standpoint. It is known in the art that increasing the operational speed of a refiner reduces the total specific energy requirements for production of somewhat similar quality pulp. High speed operation in a conventional single disc refiner is greater than 1800 rpm and typically at a range of approximately 2300 to 2600 rpm. For a double disc refiner, high speed operation is over 1500 rpm and typically at the range of 1800-2400 rpm. The higher rpm in the refiner results in what is defined as high intensity refining. Refining intensity can be expressed as either the average specific energy per bar impact or as the specific refining power. For further detailed definitions of high intensity refining, reference is made to "A Simplified Method for Calculating the Residence Time and Refining Intensity In a Chip Refiner", K. B. Miles, Paper and Timber 73(1991):9. Increasing the rotational speed of a refiner disc results in increased intensities of impacts of chips with the bars on the grinding face of the disc refiner. However, high speed refining can have the undesirable side effect of producing pulp that when further processed results in lower strength paper.
Another way of reducing energy costs in the entire paper production system is by high pressure steam recovery from the chip preheating. In conventional TMP systems, some mills require a thermocompressor or a mechanical compressor to boost the pressure of recovered preheat steam to a level necessary to supply a process demand elsewhere in the mill. Operation of the pre-heater at high pressure results in steam of sufficient enthalpy such that the recovered preheat steam may be directly employed in a given process or economically stepped down to a level necessary to meet a process demand.
The pressure on the chips during the preheating affects pulp quality. It is important to note that high pressure and high temperature are synonymous in refining because the two variables are directly related. An important factor in refining is the temperature of the wood chips prior to primary refining in relation to the glass transition temperature of the chip lignin (T.sub.g). This temperature varies depending on the species of the chip source.
Preheating at high temperatures, i.e., greater than the glass transition point with a conventional residence time softens the lignin to such an extent that the fiber is almost completely separated. The fibers separated under these high temperatures or pressures are largely undamaged, and they are coated with a thin layer of lignin which makes any attempt to fibrillate very difficult. The result la higher specific energy requirements and reduced optical properties of paper produced from the pulp.
Prior attempts have been made to reduce energy consumption by use of higher speed refiners and by manipulating chip and pulp temperatures above and below T.sub.g. PCT application WO 94/16139 discloses a low energy consumption process wherein material is fed into a primary refiner at conventional conditions of pressure. The refined pulp is then second stage refined at a temperature well above the glass transition temperature of lignin.